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The Ideal Compression for the 1999 - 2004 Mustang GT

Written By: Josh Honeycutt

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An important aspect of any engine’s performance both for power and streetability is the compression ratio. Lets find out what can change the compression ratio and what really is the best ratio for your Mustang.

What is Compression Ratio?

The compression ratio is the relationship between the amount of volume in the cylinder when the piston is at bottom dead center (BDC), and the amount of volume when the piston is at top dead center (TDC). Though this may sound confusing, it’s actually quite simple. For example, let’s say a cylinder with its piston at BDC contains 900cc of the air/fuel mixture. At TDC this mixture is compressed to 100cc. This ratio would be written as a 900:100, or 9:1, compression ratio. Different combinations of engine components can affect the compression ratio. Piston styles and combustion chamber size both directly affect your Mustang’s compression ratio. While an increase in compression can yield more horsepower, it is only beneficial in the correct application.

Mystachrome Mustang

The Ideal Compression Ratio for Your Mustang

The proper compression ratio for your mustang depends entirely on the application. A naturally aspirated engine will benefit from a bit more compression. By increasing the compression on the stock mustang GT to say 10.52 from the factory 9.66, you’ll gain roughly 5-10hp. The compression can be raised a bit further to gain more power; however, a higher quality fuel must be used, or engine knock will ensue.

For forced-induction mustangs, lower compression ratios are more suitable. The compression is raised in the cylinders as air/fuel is forced in. If the compression is raised too high, poof goes the engine. With a positive displacement supercharger such as an Eaton or Kenne Bell, an 8:1 ratio would be a good choice. However, with a centrifugal supercharger or turbo charger, the stock 9.66:1 compression ratio is more ideal. Since the boost comes in a little higher in the rpm range, a little more compression will get your mustang off the line a little quicker.

Centrifugal Supercharger

Ways to Raise Compression

There are several factors that affect compression ratio. One of those is the piston type.

  • Dish Type - Looks like a plate if you can imagine that
  • Flat Top - Just as it reads, flat as a pancake
  • Dome Type - A piston with punch, this type rises up higher into the chamber 

The more dish the piston has, the lower the compression will be. Flat tops and domed pistons increase compression of the cylinder. Ford chose a -17cc dished piston in the 99-04 mustang GT which provides the 9.66:1 compression ratio. By utilizing a flat-top or domed piston, the compression can be raised to provide a bit more ”squish”.

1999-2004 Mystachrome Cobra Mustang in Front of a Chopper

Things to Consider When Running Higher Compression

Although increased compression does provide a bit more horsepower, there are other factors to consider. Most importantly, the quality of fuel you fill your mustang with. With increased compression comes increased risk of pre-detonation, an engine killer. Make sure to use a premium fuel with a high octane rating. Also, an engine with higher compression will benefit from other supporting mods like headers, intake etc.

Mustang GT Headers

Build to Your Needs

As stated before, the compression of your mustang engine depends upon the application. If you plan on staying naturally aspirated, a slight increase in the compression ratio will definitely net some horsepower gains. If you plan on boosting to the moon, keep the compression down; no one likes a blown engine.

Fitment includes: 1999, 2000, 2001, 2002, 2003, 2004, GT